Migration can be a significant problem in the placement of expandable stents and other intraluminal devices, particularly when placed in the vascular system where the prosthesis is subject to the forces of blood flow, especially on the arterial side. Nowhere is the prevention of migration more important and more challenging than when placing a stent graft to repair an abdominal aortic aneursym (AAA) where downstream migration of the device can result in the aneursym no longer being excluded. If the aneurysm is no longer intact or subsequent rupture were to occur, the patient would then face an increased risk of death. Unlike surgically placed grafts which are sutured into place, only the radial forces of the stent would be available to hold the prosthesis into place.
To address the problem of migration, stent graft manufacturers sometimes place a series of barbs or hooks that extend outward from the main body of the prosthesis, typically at its proximal end, either by attaching them to the stent frame with solder or by some other bonding technique, or to the graft material, typically by suturing. It has been observed that sutures attaching barbed stents to the graft material are subject to breakage due in part to the flexibility of the graft material and the considerable pulsatile forces of arterial blood acting on the device. These forces have been known to directly contribute to the detachment between the graft portion and anchoring stent.
It has also been observed that barbs soldered or otherwise attached to the stent frame are subject to fracture, detachment, or other failure, especially when the forces become concentrated at a particular location along the stent graft. Unfortunately, simply making the barbs stronger to prevent fracture can result in increased damage to the anchoring tissue. Furthermore, adding rigidity to any outward-projecting barbs may compromise the ability of the device to be compressed and loaded into a delivery system. The use of multiple barbs can prevent catastrophic migration of the device, especially if there are a very limited number of barb failures. Yet, while a single barb failure should not result in the migration of the device and may not represent a problem clinically, barb fracture or failure is nevertheless currently classified as an adverse event that manufacturers seek to avoid.
One solution to address barb failure was disclosed in U.S. Pat. No. 5,720,776 to Chuter et al., depicted in FIG. 1, where the barb includes both a mechanical attachment, as well as the traditional solder bond. The mechanical attachment comprises a helical winding of the basal portion of the barb around a strut of the stent prior to addition of the solder joint to help protect the solder joint from failure. In addition, the barb is made laterally flexible to help accommodate forces acting at the anchor point. These improvements help ensure that the barb does not readily detach from the stent due to a failure of the solder joint alone. While the combination of both solder and a mechanical means to affix the barb to the stent has proved effective in most respects; however, this area of the barb remains most subject to stresses, such as from cyclic load resulting from the pulsatile action of the implant vessel. What is needed is a barb design that is better able to accommodate or distribute bending and shear stresses in order to further reduce the likelihood of barb failure due to fracture.